A cascading nuclear disaster is underway in Japan. Could it happen here? You bet.

Consider Diablo Canyon, California's most earthquake-prone nuclear power plant. It was built — on the central coast near San Luis Obispo, half way between San Francisco and LA — to withstand a magnitude 7.5 earthquake. Which sounds reassuring — until you realize that Japan's Fukushima nuclear plant, now in partial meltdown, was engineered to stand up to a 7.9, only to be hit by a 8.9 quake.

(Don't be misled by the logarithmic Richter scale; those numbers are worlds apart: A 7.5 quake is the equivalent to the force of detonating 180,000 kilotons of TNT; a magnitude 8.9 quake: 22 million kilotons — 122 times greater.)

Scarier still, Diablo Canyon was built with the Hosgri Fault, 4.5 kilometers offshore, in mind. But in 2008, scientists discovered a second major fault — the Shoreline Fault — only 1 kilometer offshore from the nuclear plant.

The federal Nuclear Regulatory Commission and the plant's operator PG&E have both declared the second fault a non-issue. But a September 2010 presentation from the California Energy Commission raises troubling questions about the second fault, reporting that its "major characteristics...are largely unknown," including "whether an earthquake beginning on the Hosgri Fault could continue on the Shoreline Fault or vice versa, causing a larger quake than if either fault broke on its own." Most troubling of the unknowns? "Whether this fault or fault displays could extend under the plant." (Emphasis added here and below.)

The commission's presentation blasts PG&E for not investigating known risks, reporting that the fault geology is "not understood sufficiently to rule out [an] earthquake directly beneath the plant," and that the utility "has not assessed the expected ground motions and plant vulnerabilities from such an earthquake."

In light of the fact that the current disaster in Japan has resulted in both earthquake and tsunami damage, it's also troubling that the commission presentation calls on PG&E and the feds to update Diablo Canyon's "tsunami hazard assessment."

(Calls to PG&E, the NRC, and the California Energy Commission were not immediately returned.)